7.5" Caliper Full Rebuild

[Jared Mk3]

Seeing as I was doing these anyway, I thought it might be nice for others to see how it's done, or at least how I do it. So, here's a small and basic guide on how to do the calipers up with new seal kits etc. In order to do a proper job, you'll need to have purchased the following parts :

2 x caliper seal kits
2 x caliper bridge seals
2 x new caliper bleed nipples
2 x new disc brake hoses and copper washers
4 x stainless caliper pistons
4 x new caliper bolts (if desired, you can reuse the old ones)

To make the job easier, you'll also need :

Brake fluid
Air compressor and nozzle gun fitting
Red rubber grease
Caliper paint (Duplicolour)
WD40/Q20
Vinegar
Hand-held metal brush (soft)
Drill and metal brush fitting (soft)

1. If the calipers are off of the car, the first thing to do is to remove the rubber brake hose and the pads and pins (if still attached). The bleed nipple should stay in for the next step. Also, if the calipers are on a running car, go to the next step.

2. In order to get the caliper pistons out, you could use one of two methods. If the calipers are still on a running car, leave the hose still attached to the brake system and take the caliper off of the swivel hub. Place an hold brake disc or a piece of wood in between the two pistons so that they don't fly out. Get someone to press the brake pedal and the pistons should push up against the disc/wood. The inner piston will push out more than the outer piston. Remove the disc/wood and take the hose off the caliper, then pull the pistons out.

If the calipers are already off the car or were bought, the easiest way to get the pistons out is to use compressed air. Take an old brake hose and cut it and fit it to the caliper. Then, using an appropriate sized nozzle gun (don't know how else to explain this particular item - I don't unfortunately have a picture) with the compressor pipe, place the nozzle into the brake hose and push air into the caliper. Again, an old disc or a piece of wood in between the pistons will help here. The pistons should push out from the pressure. If not, spray something like WD40/Q20 into the caliper from the brake hose hole and let it sit over night or a couple days, then use the compressed air again. (Just a note on this, some guys suggest using grease and filling the caliper by pumping it in until the pistons come out, but I see that being a headache to clean up.)

3. With the pistons now out, the bleed nipple can be removed. The two caliper halves can now also be split using a 9/16 socket and 'strong arm' bar. The easiest way to get this done is to place the caliper in a vice and then undo the bolts as they can be quite difficult to get undone. 

With the halves can apart, the caliper will now look like this, with the bridge seal and piston seals still in place :



4. Now to remove the bridge seals is done by simply lifting out with a flat screw driver (don't scratch the mating surfaces between the two halves). The first piston seal has a metal retaining ring which can be gently pried out with a flat screw driver. The second seal inside the piston sits inside an indentation. Again, using a flat screw driver the seal can be pushed out from the indentation and then pulled out (don't scratch the bores of the pistons). 

5. From the images above, it's clear how the piston bores can be rusty, so this next step is to remove the corrosion. A cheap and easy way to do this is to submerge the caliper halves in spirit vinegar. They can be left for about 24 hours or so to remove most of the corrosion. After this, using a soft hand-held wire brush, the calipers can be cleaned off. Use a cleaning agent like Cleen Green to remove any grease as well as the vinegar, then wash off thoroughly with water. 

The caliper pieces should now look like this :



6. Surface corrosion will start to appear after the cleaning. To get rid of this and any other hard to remove pieces of corrosion, use a soft wire brush in a drill to clean the surface up (don't use it in the piston bores). I used thinners to clean the piston bores of any surface corrosion and to wipe the calipers off to remove any dirt/dust left from the drill.

The calipers should look like this after the wire brush treatment :



7. Leave the calipers to dry out for a bit and to let the thinners evaporate. Place in an oven at around 100 degrees Celsius for about 30 mins to make sure that absolutely no moisture is left, especially in the fluid tube for the caliper bridge on each half.

8. Using the caliper paint, paint around the piston bores and in the areas around it now, because when assembled it'll be harder to reach in these areas. 2-3 coats will be sufficient (don't get paint between the two mating surfaces).



9. Now to fit the seals and pistons. The inner seal is a thick piece, thicker than the out scraper seal. Coat this inner seal in brake fluid and place it in the indentation in each piston bore. Place the scraper seal in the metal retaining ring (the ring is slightly smaller than the scraper seal, so getting the seal to sit in the ring takes a little practice and patience). Place the ring and seal in the upper groove of each piston bore. Then, using a large washer or socket, knock the ring into the groove making sure it stays square as the ring can bend/buckle quite easily.

It should now look like this :



To get the piston in, take a very small amount of red rubber grease and coat the piston. Without this grease the piston is very difficult to get into the bore :



Once the piston is in, it'll look like this :



10. Take the rubber bridge seal and coat in brake fluid, then place in the indentation of the inner caliper half, like so :



Place the outer half onto this inner half and put the bolts through. Tighten the bolts down in equal stages, so that the outer half goes down square and the bridge seal seals properly between the mating surfaces. 



11. Now, paint the calipers in 3-4 coats of caliper paint to ensure good coverage and protection.



12. Lastly, fit the new bleed nipple and brake hoses with their copper washers. 

Hopefully this guide is helpful. Obviously, this is the way I do it, so if there are any other opinions or views, add them in before make the thread a sticky.

[Vinay-RS]

Thank you so much for writing this up Jared! It is going to be so useful for anyone who needs to go through the process.

[Jared Mk3]

I hope it helps someone out there.